I'm just starting out in this brevet business, but some things are already obvious to me, such as:
1) Limit your level of effort. I won't go more than 85% of MHR = 94% LT unless I can see I'm going to make it in comfortably. I try not to go more than 78% MHR = 86% LT when I'm pulling or solo. That said, I try to start in the HR range at which I finished the previous shorter brevet. That means that I'd start a 600 trying to keep my max at 81% MHR = 89% LT. These are just my numbers. Develop your own.
2) Suck wheel, and more than that, suck fast wheel. Someone moves out and moves out solo - let them go. Your group will almost certainly have them back. Two people move out, bridge up. It's better to have riders behind you than in front of you. You can't suck a wheel that's gone. But if they push your HR higher than you want when you're drafting them, let them go. There'll be more behind you. If they push your HR too high on a climb, let them go, but don't give them up. They'll probably back way off on the descent. That's what your 52X12 is for. Keep your effort even and get back on. You'll save yourself a lot of pain and effort if you can stay with a group who might be faster than you.
3) Pull when it's your turn. I add 10 about beats to my drafting HR when I pull - that usually keeps the effort about the same even if the terrain changes. You don't have to pull long to keep your honor. Ten minutes is fine if you feel like it. So is one minute when you're tired.
4) There's strength in numbers. If you see someone getting dropped, go back and see if they want you to pull them back up. They may come in handy later. If you can't do that, your group is too fast. Ride with the droppee instead. After a hard hill or difficult rollers, see if the group will wait for a minute or two for stragglers. Again, they may come in handy later. Maybe the slower riders will get dropped for good, but you've moved them up the road a bit and they've helped you, too. Make common cause with others. It's not a race.
5) Getting gapped by one minute can turn into an hour by the finish. Imagine what your last brevet would have been like if you'd had to ride an extra hour.
6) Do whatever you must to get out of a control with the group with which you came - or with even faster riders if you can.
7) The usual stuff about eating and drinking: 250 cal/hr and a 750ml bottle/hr. 1-2 Endurolytes/hr.
8) You're not alone in your pain. You probably don't show it. They don't either.